Structural and functional characteristics of African rice (Oryza glaberrima) flour and starch

Abstract

African rice (Oryza glaberrima) and Asian rice (Oryza sativa) are two distinct types of domesticated rice with the former being less extensively studied. With the expansion of rice-based food products in Sub-Saharan Africa, characterization of African rice and their starches is essential for product development. Seven African Rice Accessions (ARAs) were evaluated for grain quality attributes, thermal and in vitro digestive properties. Additionally, the physical, molecular and structural properties of starches isolated from the ARAs were investigated. They were compared with two Asian Rice Varieties (ARVs) and NERICA 4 (cross between sativa x glaberrima). The physical grain quality characteristics of the intermediate-high amylose ARAs were comparable to the low amylose ARVs however, the ARAs had significantly (p<0.05) lower cooking solids loss and harder texture while the ARVs were softer and more adhesive. The glycemic indices of both rice types were > 70. The ARV flours showed higher breakdown tendency than the ARAs. All the starches displayed 'A' type X-ray diffraction pattern and micrographs showed tightly packed polyhedral granules. The gelatinization transition temperatures: onset (To), peak (Tp) and conclusion (Tc) were significantly (p<0.05) higher in ARA flours, and melting transition temperatures of retrograded flour gels showed significant differences (p<0.05) among samples. The energy (ΔH) required to gelatinize the starches or melt retrograded starch gels was greater in the ARAs. The relative molar concentration of B-chains in the amylopectins was approximately 50% in all samples, consequently, the ratio of A:B-chains was 1.0. The ARAs and NERICA 4 had greater amount of “fingerprint” Bfp-chains than the ARVs but the major group of short B-chains was similar in all samples. Positive correlations (p<0.05) were observed between external chain length (ECL) and To, Tp, Tc and ΔH. The average degree of polymerization (DP) of the ARA clusters were larger and the ARAs and NERICA 4 had significantly higher average DP of the branched building blocks (Bbl) than the ARVs due to abundance of large Bbl of group 6. This also resulted in higher average number of chains (NC), longer average chain length (CL) and higher degree of branching (DB) of the Bbl in the ARAs